Science - Class 8

Chapter 10 : Refraction and Dispersion of light

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A. Fill in the blanks.

1. A ray of light, passing from one medium to another, does not bend its path if its angle of incidence equals ________ degree.

ANS: 0°

2. The more is the optical density of a given medium, the ________ is the speed of light through it.

ANS: less

3. The phenomenon of the splitting of white light into seven colours is known as the phenomenon of ________.

ANS: dispersion

4. The point, on the principal axis of a concave lens, from where a beam of incident parallel ray appears to diverge, is called the ________ of the concave lens.

ANS: principal focus

5. An object should be placed at the ‘________’ point so that a convex lens forms its real and inverted image of the same size.

ANS: 2F

B. Write True or False for the following statements.

1. The basic cause of refraction is the change in the speed of light as it goes from one medium to another.

ANS: True

2. In an optically denser medium, the speed of light is more than the speed of light in vacuum.

ANS: False

3. An (obliquely) incident ray always bends away from the normal when it passes from one transparent medium into another.

ANS: False

4. A coin, kept at the bottom of an empty dry cup, appears to ‘rise up’ when some water is poured into the cup.

ANS: True

5. When white light is ‘dispersed’ by a glass prism, the yellow colour in it, bends more than the blue colour.

ANS: False

C. Tick (✓) the correct option.

1. Which of the following conditions is not necessary for a change in the direction of propagation when a light ray goes from one medium to another?

•  the two media should have different optical densities

•  the incident ray should fall obliquely on the surface separating the two media

•  the two media should be transparent

•  the incident light rays must always have only the blue colour

ANS: the incident light rays must always have only the blue colour

2. Which of the following diagrams correctly shows the ‘bending of a light ray’ as it goes from an optically denser medium into an optically rarer medium?

•  (a)

•  (b)

•  (c)

•  (d)

ANS: (b)

3. The angle, between the incident ray and the emergent ray, for a rectangular glass slab is—

•  180°

•   90°

•   0°

•   depends upon the colour of the light ray

ANS: 0°

4. Which of the following figures correctly represents the passage of white light through a glass prism?

•  (a)

•  (b)

•  (c)

•  (d)

ANS: (c)

5. An incident ray, passing through the optical centre of a concave lens, after refraction through it, will—

•  bend towards the principal axis

•  bend away from the principal axis

•  pass through the principal focus

•  go undeviated

ANS: go undeviated

6. When an object approaches a convex lens, from infinity towards its focus, the image, formed by it—

•  remains fixed

•  keeps on shifting away from the lens

•  keeps on shifting towards the lens

•  first shifts away and then shifts towards the lens

ANS: keeps on shifting away from the lens

7. Tanya’s grandmother needs a lens to read small letters in her book. For this she should use—

•   a convex lens, kept at a distance more than its focal length from the book

•   a convex lens, kept at a distance less than its focal length from the book

•   a concave lens, kept at a distance less than its focal length from the book

•   a concave lens, kept at a distance more than its focal length from the book

ANS: a convex lens, kept at a distance less than its focal length from the book

D. Answer the following questions in brief.

1. Light propagates faster through a Medium A than through another Medium B.

(a) Which of the two media has a higher optical density?

ANS: Medium B has a higher optical density because light travels slower in an optically denser medium.

(b) State the condition under which no ‘change in the direction of propagation’ of a light ray would take place when it passes from Medium A to Medium B.

ANS: No change in direction will take place if the light ray is incident normally on the surface separating the two media (i.e., angle of incidence = 0°).

2. Why does a pencil appear bent when it is partially dipped in water?

ANS: A pencil appears bent when partially dipped in water due to refraction of light. The light rays coming from the submerged part of the pencil bend away from the normal when they pass from water to air. Hence, the immersed part appears raised, making the pencil look bent.

3. Trace the path of rays in the following ray diagrams:

ANS:

•      In the first diagram, the ray enters the glass slab from air and bends towards the normal. On emerging out, it bends away from the normal and emerges parallel to the incident ray.

•      In the second diagram, the ray enters the inclined glass slab from air and bends towards the normal. On emerging, it bends away from the normal and comes out parallel to the incident ray with lateral displacement.

•      In the third diagram, the ray enters the slab obliquely from air, bends towards the normal inside the slab and bends away from the normal on emerging, finally coming out parallel to the incident ray.

4. When a spherical lens is held towards the Sun and a sharp image is formed on a piece of black paper, a hole gets burnt in the black paper, after some time.

(a) Name the lens used in the above activity.

ANS: Convex lens.

(b) What name is given to the distance between the spherical lens and the black paper?

ANS: Focal length of the lens.

(c) What is the relative advantage of using a black paper rather than a white paper?

ANS: Black paper absorbs more heat energy than white paper, so it gets heated faster and burns easily.

5. What kind of lens would form—

(a) an inverted and magnified image of the object?

ANS: Convex lens.

(b) an erect and magnified image of the object?

ANS: Convex lens.

(c) an inverted and diminished image of the object?

ANS: Convex lens.

(d) an erect and diminished image of the object?

ANS: Concave lens.

6. Draw labelled ray diagrams to illustrate the differences between the virtual images formed by using (a) a concave lens (b) a convex lens.

ANS:

(a) Concave lens: A concave lens always forms a virtual, erect and diminished image between the optical centre and focus, on the same side of the object.

(b) Convex lens: A convex lens forms a virtual, erect and magnified image when the object is placed between the optical centre and the principal focus. The image is formed on the same side of the lens as the object.

E. Answer the following questions.

1. A coin is placed at the bottom of a clear glass vessel. An observer moves herself to a position where the coin goes just out of sight of her eyes. Keeping her eyes in that position, she asks her friend to pour some water, gradually into the glass vessel.

(a) What would she observe?

ANS: On pouring water into the vessel, the coin becomes visible again and appears to rise up.

(b) Name the phenomenon involved in this observation.

ANS: Refraction of light.

(c) Draw a neat labelled diagram to justify your answer.

ANS: Draw a ray diagram showing:

•      coin at the bottom of the vessel,

•      water filled in the vessel,

•      ray from coin refracting at water-air surface,

•      refracted ray reaching observer’s eye,

•      apparent raised position of coin.

2. In the following figures, identify the light ray that would be the correct emergent/refracted light ray in each case. Give reason for your answer in each case.

ANS:

•      Fig (a): Q is the correct emergent ray, because in a rectangular glass slab the emergent ray is parallel to the incident ray.

•      Fig (b): P is the correct refracted ray, because light bends away from the normal when it passes from air to water is false; here correct bending is towards the normal in denser medium, so the suitable ray is P as shown.

•      Fig (c): R is the correct emergent ray, because light passing from glass to air bends away from the normal.

3. White light is made to fall on one face of a glass prism. Draw a ray diagram to show the nature of the emergent beam.

ANS: Draw a triangular glass prism showing:

•      white light incident on one face,

•      dispersion inside the prism,

•      emergent spectrum of seven colours (VIBGYOR),

•      red deviated least and violet deviated most.

4. With the help of a suitable diagram, explain the meaning of terms:

(a) centres of curvature

ANS: The centres of the two spheres of which the two curved surfaces of a lens are parts are called the centres of curvature.

(b) optical centre

ANS: The optical centre is the point on the principal axis of a lens through which a ray of light passes undeviated.

(c) focus

ANS: The focus (principal focus) is the point on the principal axis where rays parallel to the principal axis converge (convex lens) or appear to diverge from (concave lens) after refraction.

5. We are given a convex lens of focal length 20 cm. Draw ray diagrams to show the nature, size and position of the image formed when the object is kept at a distance of

(a) 55 cm

ANS: Object is beyond 2F. Image is formed between F and 2F on the other side; real, inverted and diminished.

(b) 35 cm

ANS: Object is between F and 2F. Image is formed beyond 2F on the other side; real, inverted and magnified.

(c) 15 cm

ANS: Object is between optical centre and F. Image is formed on the same side of lens; virtual, erect and magnified.

6. We are given a concave lens of focal length 15 cm. Draw a ray diagram to show the nature, size and position of the image formed when the object is kept at a distance of

(a) 30 cm

ANS: Image is formed between optical centre and F; virtual, erect and diminished.

(b) 15 cm

ANS: Image is formed between optical centre and F; virtual, erect and diminished.

(c) 10 cm

ANS: Image is formed between optical centre and F; virtual, erect and diminished.

7. What type of a lens is used as a ‘magnifying glass’? How is the object positioned with respect to this lens? Draw the appropriate ray diagram.

ANS: A convex lens is used as a magnifying glass. The object is placed between the optical centre and the principal focus of the convex lens. It forms a virtual, erect and magnified image on the same side of the lens as the object.